code: def test_loss(self, mock_cross_entropy):
      mock_cross_entropy.return_value = 1.0
  
      loss = Mixup.mixup_loss
      res = loss(
          {
              torchbearer.DATA: torchbearer.TRAIN_DATA,
              torchbearer.Y_PRED: "test1",
              torchbearer.Y_TRUE: ("target1", "target2"),
              torchbearer.MIXUP_LAMBDA: 0.7,
          }
      )
  
      mock_cross_entropy.assert_has_calls(
          [call("test1", "target1"), call("test1", "target2")]
      )
      self.assertTrue(res == 1.0)
  
file_path: torchbearer/tests/callbacks/test_mixup.py
imports: from unittest import TestCase
  from mock import patch, call, MagicMock
  import torch
  import torchbearer
  from torchbearer.callbacks import Mixup, MixupAcc
label_desc: Use image node_id 5 for calling the TestMixupInputs obj's underlying member method code with example usage: obj.test_loss(mock_cross_entropy) without return types